SCH3U Lewis Structures Half Notes
Molecular Shapes and Polarity
• Bond polarity is a measure of how equally the electrons in a bond are
shared between two atoms. As the difference in electronegativity between
two atoms increases, so does bond polarity.
• In molecules containing two atoms, a dipole exists if there is a difference
in electronegativity.
• For a molecule with more than two atoms, the dipole moment depends on
both the polarities of the individual bonds and the geometry of the
molecule.
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Consider linear CO2
Each C=O bond is polar, but since the bonds are identical, the bond
dipoles are equal in magnitude. The overall dipole moment = 0.
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Bond dipoles and dipole moments are vector quantities, which means they
have magnitude and direction. The overall dipole moment of a molecule is
the sum of its bond dipoles.
Consider the bent molecule, water, with two polar bonds.
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Both bonds are identical so the bond dipoles are equal, but since the
molecule is bent, the bonds do not directly oppose each other. Therefore,
the bond dipoles do not cancel each other out.
The water molecule has a non zero dipole moment so it is polar. The
oxygen carries a partial negative charge and the hydrogen atoms each
carry partial positive charges.
SCH3U Lewis Structures Half Notes
Comparing Models
Comparing Models
SCH3U Lewis Structures Half Notes
Drawing Lewis Structures
1. Arrange the element symbols.
Central atoms are generally those with the highest bonding capacity.
Carbon atoms are always central atoms
Hydrogen atoms are always peripheral atoms
2. Add up the number of valence electrons from all atoms.
Add one electron for each negative charge and subtract one for each positive
charge.
3. Draw a skeleton structure with atoms attached by single bonds.
4. Complete the octets of peripheral atoms. Hydrogen will not have any lone
pairs!
5. Place extra electrons on the central atom.
6. If the central atom doesn’t have an octet, try forming multiple bonds by
moving lone pairs.
Structural Formula
7. From the Lewis structure, remove dots representing lone pairs
8. Replace bond dots with a dash
CCl2F2
NH3
CCl4
CH4O